1. Field of the Invention
The invention relates to a backlighted liquid crystal display system with uniform brightness of the viewing area.
2. Description of the Related Art
Liquid crystal displays (LCD's) are commonly used in portable computer systems, televisions and other electronic devices. An LCD requires a source of light for operation because the LCD is effectively a light valve, allowing transmission of light in one state and blocking transmission of light in a second state. Portable and laptop computers typically use an LCD panel that comprises a liquid crystal polymer encapsulated between at least two planar glass plates in parallel with each other. A polarization layer is bonded to the outer surface of each glass plate such that the glass plates are sandwiched between two polarization layers. An alternate type of LCD panel may include a thin polymer film, or retardation layer, located between each glass plate and its respective polarization layer.
The inner surface, or the surface facing the liquid crystal polymer, of each glass plate includes mutually perpendicularly oriented conductive transparent lines. The volume between any two orthogonal lines forms a cube whose face area constitutes a pixel. The lines are connected on the periphery of the glass plates via input/output (I/O) strips to accompanying electronic circuitry. The functions of the LCD panel elements introduced above will not be described as they are well known by those skilled in the art.
Backlighting the LCD panel has become the most popular source of light in portable computer systems because of the improved contrast ratios and brightnesses possible. A backlight including, a light source and a light pipe located next to and aligned parallel with the back surface of the LCD panel provides light to the LCD panel. The light pipe is capable of reflecting are distributing the light perpendicularly over its entire front surface, wherein the front surface was either optically bonded or placed adjacent to the back surface of the LCD panel. The light source is conventionally a florescent tube attached to at least one edge surface of the light pipe. A reflective material may be wrapped around the light source to redirect light from the light source into the light pipe.
The brightness of the LCD panel is related to the transmission characteristic of the liquid crystal polymer and the retardation layer polymer. The LCD panel is temperature sensitive in that the transmission characteristic of the liquid crystal polymer and retardation layer polymer changes significantly with variations in temperature. A uniform transmission characteristic over the LCD panel requires as uniform a temperature distribution as possible. The main drawback of the assembly described above is that the heat dissipated by the light source(s) is either radiated directly or conducted via the light pipe to the LCD panel. The undesirable result is a temperature gradient between the back and front surfaces as well as a temperature gradient between the edges and the center of the LCD panel. These temperature gradients cause the LCD display to have different brightnesses around its edges as compared to its center so that the viewing area does not have a uniform brightness.